The present invention relates to a process and an apparatus for coating a slurry coating material onto a continuous base sheet at a given thickness, and particularly to a process and an apparatus for intermittently coating a slurry coating material to alternately form coated areas and uncoated areas, each having a predetermined length on a base sheet.
In mass production of, for example, spiral electrode type lithium batteries, the following intermittent coating technique is employed. A band-shaped hoop material of a copper or an aluminum foil approximately 400 mm wide and 10 .mu.m thick is employed as a base sheet for current collectors for such batteries. Slurry mixture containing electrode active material as a main component is used as a coating material. The coating material is applied onto the base sheet, for example, over areas each having a length of 30 cm at a certain thickness, in such a manner that the coating material is not applied at all to subsequent areas, each having a length of 5 cm. Thereby, the coated areas and the uncoated areas, each having the above-mentioned lengths, are alternately formed on the continuous base sheet. Finally, the base sheet is cut at the uncoated areas into a plurality of electrode sheets for the batteries. Each electrode sheet is wound with another electrode sheet constituting another electrode, with a separator therebetween. Then, the resultant wound electrode body is assembled in a battery case.
One known process for carrying out such an intermittent coating operation employs a coating apparatus called a reverse roll coater as shown in FIGS. 10 to 12.
In FIG. 10, the coating apparatus includes, as principal portions, a supplying drum 10 for supplying a base sheet 3, an intermittent coating portion 20 for intermittently applying coating material 6 onto the base sheet 3, a drying furnace 21 for drying the coated material 6 and a take-up roll 22 for taking up the base sheet 3 having passed through the drying furnace 21. As shown in FIG. 11, the base sheet 3 supplied form the supplying drum 10 is guided to the intermittent coating portion 20 by guide rolls 11a, 11b, feed rolls 9 and a guide roll 8.
In the intermittent coating portion 20, as shown in FIG. 12, a coating roll 1 (hereinafter referred to as a C-roll) is rotated at a predetermined speed in a direction indicated by an arrow a, whereas a backing roll 2 (hereinafter referred to as a B-roll) is rotated in a direction opposite to that of the C-roll 1 (i.e., in a direction indicated by an arrow b). The base sheet 3 is allowed to travel in a direction indicated by an arrow c along the B-roll 2 and to pass between the C-roll 1 and the B-roll 2.
An adjuster called a doctor roll 4 (hereinafter referred to as a D-roll) is fixedly (i.e., non-rotatably) mounted above the C-roll 1 and spaced a predetermined distance from the C-roll 1. The coating material 6 accumulated in a hopper 5 is deposited onto a peripheral surface of the C-roll 1, then passes through a region of minimum distance between the C-roll 1 and the D-roll 4, whereby the amount of the coating material 6 applied to the base sheet 3 is controlled at a certain value (in other words, thickness) corresponding to the minimum distance.
A coating layer 6a controlled to the predetermined thickness by the distance between the C-roll 1 and the D-roll 4 is transferred onto the base sheet 3 when the base sheet 3 passes between the B-roll 2 and the C-roll 1. In this manner, a coating layer 6b is continuously formed on the base sheet 3 at a controlled constant thickness.
In order to form uncoated areas intermittently, there is mounted an intermittent moving mechanism for moving the B-roll 2 in a direction perpendicular to a rotational axis thereof. Thus, the B-roll 2 is displaced in a direction indicated by an arrow d at given intervals, until it is sufficiently spaced apart from the C-roll 1 so that the coating layer 6a is not transferred from the C-roll onto the base sheet 3. After a lapse of a predetermined time, the B-roll 2 is moved in a direction indicated by an arrow e back to its original position, and the coating layer 6a on the C-roll 1 is again transferred to the base sheet 3. By repeating these operations, the above-described intermittent coating can be achieved. Then, the intermittently coated base sheet 3 is led over a guide roll 7 to the drying furnace 21. Eventually, the base sheet 3 is taken up by the take-up roll 22 after passing through the drying furnace 21.
However, the above-mentioned existing intermittent coating process may suffer from a problem that it might be almost impossible to accurately apply the coating layer because of unstable travel of the base sheet 3 due to the movement of the B-roll 2 during a transition period from a coating operation to an operation of interrupting coating. When the B-roll 2 is displaced by the C-roll 1, the base sheet is loosened between the guide rolls 7 and 8 since such guide rolls are fixedly disposed. A certain tension is applied to the base sheet 3 by the take-up roll 22, and therefore this looseness usually disappears in short time (for example, in less than half a second). However, since the respective distances between the B-roll 2 and the guide rolls 7 and 8 are short, the angles of deflection of the base sheet 3 spanning rollers 2, 7 and 8 at the points of contact with the rollers 7 and 8, become relatively large. Then, resumption of tightening of base sheet 3 along the B-roll 2 sometimes causes wrinkles or folds around the previously loosened portion of the base sheet 3. Due to these wrinkles and/or folds, the coating material 2 cannot be uniformly applied to the base sheet 3, and a non-uniform coating may be applied thereto. In a mass-production line for the aforementioned spiral type electrode batteries, for example, the following deficiencies may result due to the non-uniformity of coating. Firstly, a paste tends to partially drop off from the sheet at thick coated portions. Secondly, at thin coated portions, a current collector made of a metal foil often may be broken apart by excessive partial rolling. Furthermore, the non-uniform coating may cause incorrect winding during the step of winding the sheet-shaped electrode with a separator into a spiral shape. These phenomena will prevent efficient manufacturing of the batteries. Additionally, even after assembled as complete batteries, optimal electric capacity balance will not be obtained. Specifically, for spiral lithium secondary batteries, active material will not be efficiently available at the non-uniformly coated portions. When there are thick coated portions only on the positive electrode sheet in the secondary batteries, the capacity of the negative electrode becomes relatively smaller than that of the positive electrode. As a result, dendrites may grow at those portions of the negative electrode when the batteries are charged and such dendrites may cause short-circuiting between the two electrodes.
The present invention has been accomplished under such a technical background, and it is an object of the present invention to provide an intermittent coating process and an apparatus therefor, whereby it is possible to ensure stable travel of the base sheet without deformation and to prevent non-uniform coating due to wrinkles or folds in the sheet over the entire coated area.